Gas heating furnace with tubular heat exchange means



3 Sheets-Sheet l J. O. NAUCLER I'AL `April 24, 1956 GAS HEATING FURNACEWITH TUBULAR HEAT EXCHANGE MEANS Filed May 3l, 1950 April 24, 1956 J. o,NAUCLER ErAL 2,742,895

GAS HEATING FURNACE WITH TUBULAR HEAT EXCHANGE MEANS Filed May 5l, 19503 Sheets-Sheet 2 mm, Mm? ffm TOP/VEYS' J. O. NAUCLER ET AL April 24,1955 GAS HEATING FURNACE WITH TUBULAR HEAT EXCHANGE MEANS 3 Sheets-Sheet3 Filed May 5l 1950 l l l l i f l f f f l f l f C emar/'ei mi j y Perc/oz an fno' United States Patent GAS HEATING FURNACE WITH TUBULAR HEATEXCHANGE MEANS Application May 31, 1950, Serial No. 165,202 In SwedenNovember 28, 1941 Section 1, Public Law 690, August 8, 1946 Patentexpires November 23, 1961 22 Claims. (Cl. 12b-1M) The present inventionrelates to furnaces for heating owing gases mainly by radiation fromgaseous heat sources of relatively high temperature, and moreparticularly to radiation gas heaters of the kind in which the gases tobe heated are passed through vertical tubes disposed in a ring in avertical refractory shaft and forming a central vertical passage for theheating gas.

Gas heaters of this general character are known in the art, and similarstructures are widely used for steam boilers and also employed in theart of cracking oils. However, in spite of the various precautions thathave been resorted to for protecting the heating surfaces againstdeterioration, none of the devices heretofore known has been capable ofusing heating gases of very high temperature and at the same timeattaining a high degree of heat transfer eiliciency. Y

Itis therefore a primary object of this invention to provide a tubulargas heater that will offer a very high efciency per unit of heattransmitting surface when using heating gas of a high temperature andmake it possible to utilize the tube material to the utmost of itscapacity.

It is also an object to provide a structure that will toleratepractically any attainable temperature of the heating gas and whereinpractically the only factor on which the heat transfer is dependent isthe quantity of heat supplied.

Another important object of the invention is to provide a gas heaterstructure that will be easy, practical and economical to build and sodesigned that substantially the entire tube system may be manufacturedas a unit at any distance from the place where the gas heater is to beerected.

These and other valuable objects that will be evident as the descriptionof the invention proceeds will be obtained primarily by arranging thetubes in spaced relationship vertically in a single ring Abetween upperand lower head boxes to form a tube-cage which is handleable as a unit,placing thiscage within the heat transmission space in the shaft so asto freely depend from a support for the upper head box, and covering theshaft by a refractory head-unit containing a vertical passage for theheating gases, the gas heater thus being composed of three separateunits: the shaft, the tube-cage and the head-unit.

For a better understanding thereof, the invention will now be describedmore in detail with reference to the embodiments illustrated in theaccompanying drawing, although it will be understood that the inventionis not limited to these disclosures'which are illustrative only.

Inthe drawing, wherein like reference characters indicatev like partsthroughout the several views:

Fig. 1 is a vertical section of one form of radiation gas heaterembodying the invention.

Fig. 2 is a horizontal section taken along the line 2 2 in Fig. l.

Fig. 3 is a fragmentary elevation, partially in section, of an expansiondevice alternative to that shown in Fig. 1. Fig. 4 is a vertical sectionof a second form of gas heater including diagrammatic showingsautomatically controlling the supembodying the invention, of alternativemeans for ply of the heating gas.

As will be seen from Fig. l, the heat exchanger consists of three mainparts: a shaft formed by the heat resisting wall 7, a tube-cageconsisting of the tubes 6 and the upper and lower head boxes 10 and 11,and the head-unit 12 constituting a cover for the shaft and containingthe heating gas inlet passage 2 which is coaxial with and communicatesdirectly with the central unobstructed portion of the shaft. The shaftwall 7 is built up from different layers, such as, in order from within,refractory bricks, chamotte grains, rock wool and outermost a casing ofiron plate.

The shaft has an airtight bottom 25 and rests upon a support S. Throughthe bottom projects a vertical ilue 9 which is coaxial with the shaftand provides a passage for the heating gas leading to the outlet conduit5.

Suspended in the shaft is the tube-cage consisting of an annular upperhead box 10, a ring of tubes 6 secured to and opening into said box 10,and an annular lower head box 11 secured to the lower ends of tubes 6and into which the latter all open. In the form shown, the upper headbox 10 is substantially triangular in cross section with a at bottom,while the lower head box 11 is rectangular in cross section. In order toobtain the most efficient heat transfer by radiation from and through alarge bulk of gas, the length of the tubes 6 should be less than threetimes the diameter of the ring in which they are arranged. The tube-cageforms a removable unit which is supported in the shaft by means of aconical plate 26 having its lower edge resting on the T-iron 27 which isin turn supported by the iron plate casing of the shaft wall 7. Theplate 26 is an integraly extension of, or may be fixed to, the outerinclined side of head box 10. Alternatively, the bottom may restdirectly on the upper end of the brick layer of the wall 7 which forms ashelf 29 extending slightly above the rest of the shaft. The tubesshall, however, suspended freely from the upper head box. It is alsowithin the scope of the invention to subdivide the tube-cage intovertical sections, a construction which is particularly advantageous inVeryflarge heaters.

By providing the upper head box 10 with a flat bottom the tubes can beattached to said bottom near its inner edge and a large portion of thebox can overlie the shaft wall 7 or shelf 29, either to rest thereon ormerely to be shielded or protected thereby from radiation. The other orinclined sides of the upper head box are also relatively flat whichfacilitates manufacture of the box, particularly when its diameter is aslarge as about 2 meters, for example. Eyes 28 may be welded or otherwisefixed-to the head box 10 so that the entire tube-cage can be lifted outof the shaft or lowered therein as a unit. The weight of the tube-cageensures a perfectly airtight engagement be'i tween the supporting parts26 and 27.

The shaft is limited at the top by the head-unit 12 coninclined wall ofthe upper head box 10 and is preferably held in gastight relationtherewith solely by its own weight, although special means may beemployed for the latter purpose, if desired. In the form shown, thehead-unit 12 is provided with connections 3 a manner known to the art(cf, Patent No. 95,240). After substantially complete combustion, thegases reach the radiation space 1 at the upper radiation zone will bemoved to a lower level in the shaft.

It will be apparent to those skilled in the art that, if desired, thegas inlet passage in the head-unit may be connected to any suitablesource of hot gas instead of being combined with aburner as indicated inPig. l.

It is also obvious that the head-unit may be supported by the wall 7directly, or by the intermediation `of means other .than the upper headbox 1G, without departing from the invention. On the other hand, it isimportant that the head-unit be a separate structure which is removableas a unit, but at the same time supported by the wall 7 so that it mayfollow the thermal movements thereof and thereby facilitate maintenanceof .a gastight connection between the head and the shaft. The.removability of thc head enables ready access to the interior of theshaft and facilitatesl insertion and removal of the tube-cage.

Since, in a heater wherein the heating gas is introduced at the top, thestrongest radiation emanates from the portion of the shaft marked 1,situated immediately below the gas inlet 2, and from adjacent surfacesof the headunit, the upper portions of the tubes 6 are exposed to Vthegreatest strain. Therefore it is particularly important that the upperportions of the tube system be properly shaped and arranged and, as thecase may be, carefully protected. ln the apparatus shown in Fig. l, thedesired result is attained partly by suitably shaping the head-unit sothat it is provided with projecting portions 17 which screen off theupper head box 10, and partly by arranging a considerable portion of thebox 10 above the wall 7 so as to be screened thereby.

The tubes 6 are arranged in a single ring along the cylindrical shaftwall 7 with suicient distance between the tubes and the wall (e. g., 1to 4 decimeters), as well as between adjacent tubes (preferably 1/2 to 4tube diameters, e. g., l to 2 tube diameters), to allow the portions ofthe shaft wall beyond the tubes to be subjected to radiation from theheat source in the center of the shaft and thereby attain such a hightemperature that they, in turn, can effectively emit radiation to therear or outer surfaces of the tubes and thereby ensure a more uniformheating of the latter.

As shown, the tube-cage tapers downwardly and the lower head box 11 isof smaller diameter than the upper box 10, a construction whichfacilitates insertion of the cage into the shaft and its verticalmovement therein or removal therefrom, and also permits the tubes toexpose a greater surface to the hot gases entering at the top. ln orderto obtain the desired tapered arrangement, the distance between thetubes and the shaft wall may be, for instance, 1 to 2 decimeters at thetop and 1.5 to 4 decimeters at the bottom in a furnace having a heightof 4 to 6 meters.

The tube-cage comprising the upper and lower head boxes 10 and 11 andtubes 6 is preferably welded together, an operation which can be carriedout in any suitable work shop regardless of its distance from the placewhere the gas heater is to be erected. In case atempering furnace largeenough to house a structure of this size should not be available, thetempering may take place carefully after the cage has been mounted inthe shaft. A welded tube-cage is more heat resisting and safer than astructure embodying tube ttings of other kinds, and can consequently beused at higher temperatures than the latter.

In the preferred embodiment, the tubes are :straight throughout theirentire length and are attached tov the bottom of the upper head box andto the roof of the lower head box, thus facilitating an evendistribution of the gas and forming a structure of minimum heatsensitivity. Such an arrangement also makes it possible to utilizepractically the entire length of the tubes for Vheat transmissionpurposes. If desired, the tubes may be provided with insertions of thecharacter indicated at 18 (Fig. 2) in order to speedup the transmissionof heat from the tube walls 'to Vthe gas owing therein.

The gas to be heated may be introduced at the top into box 10 anddischarged at the bottom from box 11 or, vice versa, introduced into thelower box' 11 and removed from the upper box 10. Due to the fact thatthe tubes must support not only their own weight but also that of thelower head box 11 and the discharge pipe or pipes 14, and since thestrength of tubes decreases as the temperature rises, the gas to beheated is preferably introduced at the top, since in such case the tubeswill be effectively cooled at the end of greatest strain. This isparticularly important when, as in the preferred embodiment, the heatinggas is also introduced at the top of the shaft and the radiationconsequently is strongest there. Parallel flow of the heated and theheating gases coutributes to attainment of an even temperaature alongthe tubes, so that the tube material can be eciency utilized.

Accordingly, in the structure of Fig. 1 the gas to be heated enters theupper head box 10 through pipe 13 and is evenly distributed to the tubes6 through which it flows downwardly to the lower head box 11. The heatedgas collected in the lower head box 11 is discharged therefrom throughpipe 14 which passes through the bottom 25 of the shaft and is connectedto an expansion device 16 lead'- ing to the discharge main 15. Theapparatus is preferably provided with a plurality of such dischargepipes 14, although the number is limited to, for instance, three so thatthe number of openings in the bottom 25 may not become too great. Byusing a plurality of pipes 14 Symmetrcally disposed, the cross sectionof the head box can be reduced, the gas distribution improved anddistortion due to heat movements of the tubes avoided. When .severaldischarge pipes 14 are employed, they may be connected to an outerheader which, in turn, may be connected to the discharge main 15 in anyconventional manner.

The expansion device 16 consists of a sleeve 31 of extensiblebellows-like construction rigidly secured at its ends to flanges 32 and33 formed on the vertical portion of discharge main 15 and on pipe 14,respectively, inside of which sleeve the lower end of expansion pipe 24moves telescopically. Packing material 34, such as as bestos, is placedaround the joint between the device 16 and the pipe 14. Alternatively,the expansion device may be shaped as a kind of a liquid lock or liquidtrap wherein the liquid consists of a medium which is uid, butsubstantially non-volatile, at working temperatures, e. g. lead, leadalloys, Babbitt metal or the like. Such an embodiment is illustrated inFig. 3 in which the pipe 21 (having in some large plants a diameter ofmore than 0.5 meter) extends from the lower head box -11 and submergesin the liquid which is held in the annular con tainer or pocket 23attached to the upepr portion of the outlet pipe 22.

It will thus be seen that the ilow of gas in the tube system is dividedinto four separate zones: the distribution zone 10, the radiation zone6, the collecting zone 1I and the discharge zone 14; if desired, a fifthor secondary collecting zone may be added. Due tothe fact that the rstthree zones, the main zones, communicate directly with one another andstand in rectilinear relationship to one another, this zone arrangementcontributes greatiy to attainment of the objects of the invention, as bymaking it ypossible to use the above described unitary tubecagestructure and consequently to manufacture a substantial part of theheater as a unit in a factory. Furthermore, a very quick, one-passtransport of the gases through the heater tubes is thereby attainable, aresult which is :sought by the present invention partly in order toeffect a sufficient cooling of the tubes to make hem capable ofexracting the great quantities of Vheat desired, and partly in order toobtain as even a distribution of the gases as possible so that the gaswill have the same ini-tial state in all tubes, which is also important.

Being dependent in the shaft, the tube-cage can expand freely, `eventhough equipped at the bottom 'with the large head box and the dischargepipes attached thereto. When the gas to be heated is introduced at thetop and thus effectively cools and the upper parts of the tubes, theexpansion will be concentrated at the bottom, in which even it isparticularly important that the tubecage be suspended from its upper endand free to expand at its lower end.

Centrally of the shaft bottom 25, Ythe outlet ilue 9 for the heating gasprojects into the shaft chamber 20 to a level above the lower head box11. By this arrange ment, the flowing stream of heating gas will notdirectly contact with the head box, although the gas can neverthelessradiate to the tubes along their entire length down to the head box, andno part of the valuable tube lengh will be lost for the heat transfer,the tube material instead being utilized to the utmost.

The heat transmission preferably should take place only through thetubes and not to any material extent through the head boxes. To thisend, in addition to screening the upper head box by the projectingportions 17 of the head-unit and by the wallA 7, the lower head box 11may be protected from the radiation by ceramic or other refractorybricks 30 which are'light and loosely placed on the head box and onshelves on the wall 7 and the outlet llue 9, respectively, so that theydo not interfere with the heat responsive movements of the tube-cage.

As the heat transmission surface of the gas heater is the product oftube diameter, tube length and number of tubes, and as the tube diameterand the number of tubes cannot be increased above a certain maximum, ademand for greater heat transmission surface must be met with anincrease -in length of the tubes. But due to the fact that the tubes ofthe present form of heater are heavily strained by the load of the lowerhead box in addition to the load of the tubes themselves, a relativelylow limit is also set for the length of the tubes (i. e., less thanthree times the diameter of the tube-cage). Consequently, in order toapply the present invention to an installation requiring more heattransmission surface than can be furnished by a tube-cage of the maximumpermissible length, without the necessity for employing two separate gasheaters, the structure illustrated in Fig. 4 has been provided, whereintwo tube-cages are arranged one above the other in the same heatershaft.

In the heater of Fig. 4, the wall 7 maybe built up, in principle, in thesame way as that described with reference to Fig. l, and the uppertube-cage 41 is suspended from an upper shelf 42 adjacent the top of theshaft in substantially the same way as the tube-cage of Fig. l.

In this embodiment, the upper head box 10 which hangs on the plate 40 isrectangular in cross section and has a gas inlet pipe 47 which extendshorizontally through the upper end of the wall 7. Since the space belowthe upper tube-cage is occupied by the lower tube-cage 43,

the discharge pipe 44 from the lower head box 11 of the upper cage(which in this instance is circular in cross section) passeshorizontally through avertically elongated opening 45 in the wall 7,said opening being high enough to permit the pipe 44 to move verticallyas a consequence of the thermal movements of the tubes 6. The portion ofthe opening 45 around the pipe 44 may be sealed against the escape ofgas in any convenient manner known to the art. The head-unit l2 rests ona U-bar 39 carried by the plates 40 which rest on ring 38 lying on wall7.

The lower tube-cage 43 is suspended from a shelf 46 formed in the wall 7at the waist portion of the heater shaft, the upper head box 10 of saidcage resting on said shelf and having a pipe 47 connected thereto whichextends outwardly through an opening 48 .in the wall. Since the box 10has practically no vertical movement relative to the wall, the opening48 need be very little larger than the pipe 47. The tubes 6 and thelower head box 11 of the lower tube-cage are freely suspended from theupper head box 10 as described above. In the event that insuficientspace is available for accommodating gas supply or discharge connectionsbelow the gas heater, the connecting pipe 44 of the lower head box 11may-be passed through an opening 50 in the wall 7, as shown, similarlyto the corresponding element of the upper cage 41. This connection can,of course, be passed vertically through the bottom of the shaftsimilarly to the construction of Fig. l, if space is available.

By the illustrated arrangement of connecting pipes 44, 44', 47 and 47extending through the wall 7, the gas to be heated can be passed throughthe heater in any one of a plurality of alternative paths, inasmuch asanyone ofthe connecting pipes can be made the inlet and either of theconnecting pipes of the other tube-cage can be made the outlet, theremaining pipes then being interconnected. For example, as indicated inFig. 4, pipe 47 connected to the upper head box 10 of the lower tubecage43 may be the inlet for the gas to be heated and pipe 44 connected tothelower head box 11 of the upper cage 41 may be the outlet, the pipes44 and 47 then being interconnected as shown to lead the gas dischargingfrom the lower box ofthe lower cage to the upper box of the upper cage.In case it is desired that the lower head box 11 of the upper cage 4l bepermanently connected to the upper box 10 of the lower cage 43 so thatthe gas may flow straight through the two units, these boxes arepreferably interconnected inside the shaft so as to eliminate theopenings 45 and 48 through the shaft wall.

A shielding structure 51 is provided within the waist portionof theheater shaft in order to protect the head boxes l1 and 10 of the upperand lower tube-cages, respectively. The shield consists of a carrier:'52, as of sheet iron, which is provided at the bottom with an inwardlyprojecting flange 53 adapted to carry the refractory brick structure 57and at about the middle with an outwardly projecting bracket S4 adaptedto rest on the upper head box 10 of the lower cage. By the latterarrangement, no bracket need be interposed between the lower head box 11of the upper cage and the upper head box 10 of the lower cage, theheight of the gas heater thus being reduced by the height of such abracket. The refractory brick portion 57 of the shield is shaped toprotect the carrier 52 from radiation and impingernent of gas, a recess55 being provided at the lower edge thereof to `house the flange 53.

Light refractory bricks 3i) rest partly on the lower head box 11 andpartly on the shield 51 and a shelf 56 on the wall 7, respectively. Thebox 11 is thereby protected without impeding its vertical movement,while the inner ring of bricks 30 also protects the upper portion ofcarrier 52.

By the double tube-cage arrangement described, several advantages aregained. Apart from the more or less obvious saving in floor area andmaterial for building the shaft as well as in piping for transport ofthe gas, the provision of a second tube set below the first one in thesame shaft therewith, rather than laterally thereof in a separate shaft,offers a greater cooling surface to receive radiation from the zone ifrom which the strongest radiation emanates, in that a part of theradiation from said zone passes through the central passage of theshield 51 and reaches the tubes of the lower set. in the structureillustrated in Fig. l, a great portion of the radiation entering theoutlet flue 9 is reflected or reradiated to the tube-cage andparticularly to its upper portions near the hot zone 1, which portionsof the tubes are already heavily strained due to direct radiation fromsaid zone. Such reradiation is obviated, or at least minimized, in adouble cage structure like that of Fig. 4. Another advantage resides inthe fact that the shield 51 serves as a recollector for the verticalstream of heating gas so as to impede this gas from impinging directlyon the tubes, which would otherwise'tendto occur if the tubes were verylong. The double cage arrangement is also favorable because it `enablespassage of the gas to be heated through Vthe heater Aarbitrarily in avariety of diiferent combinations of parallel and countercurrent flow.Usually the best mode of heating is, as shown in Fig. 4, to introducethe heating gas at the top of the shaft and to introduce the gas to beheated into the upper head box of the lower cagei43; after passing thegas beinghcated through the lower tube-cage and extracting it from thelower head box l1, it is conveyed to the upper head box 10 of the uppercage 41 and, after passing through this cage, is discharged from thelower head box 11 thereof. However, it is often advantageous tointroduce the heated gas into the lower head box of the lower cage, soas to pass it countercurrcntly to the heating gas in this cage. A stillfurther advantage of the subdivision of the tubes into two cages isthat, being not so heavily strained as the upper one, the lowertube-cage can be made of a cheaper material than the upper one.

,inasmuch as gases have a very poor cooling effect, head boxes of thecharacter herein disclosed are apt to be badly affected by the heat towhich they are subjected, particularly at the places where the tubes arewelded to the boxes` According to the present invention, however, thehead boxes may be effectively protected against overheating by providingchoking or baille plates 61 inside the boxes over and near the mouths 62of the tubes 6. The plates 61 are secured to the walls of each head boxby holders 63 attached to parts of the walls which are not exposed todirect radiation from the heat source or, generally, are not subjectedto too much heat. The plates are so shaped and positioned near the wallsof the boxes ,as to form narrow passages 64 through which all the gasbeing heated is forced to flow. The quantity and velocity of the gaspassing a given area of the box wall surface are thereby greatlyincreased, a result which will in turn many times multiply the coolingeffect of the gas. Although the choking plates 61 are shown only in theupper and lower head boxes of the upper tube-cage 41 of Fig. 4, it willbe understood that similar plates may be embodied in the head boxes ofthe lower cage 43. and also in either or both of the head boxes of Fig.l.

The choking plate arrangement also improves the even distribution of thegas to the various tubes so as to assist in rendering equal the heattransmission status of all tubes, which is one of the main objects ofthis invention.

lt is common in heat exchangers to control the supply of heating gas, orgaseous fuel and combustion air, in accordance with variations in thetemperature of the heated gas. This may be carried out automatically byproviding impulse means in the exit line of the heated gas connected tomeans for controlling the supply of heating medium, whether in the formof heating gas or as one or both components of combustion. However, inthe present gas heater, where such high temperatures and such greatquantities of heating gas are employed as will strain the tubes to theutmost of their capacity, a control based on the temperature of theheated gas is usually not sufficient. Accordingly, another feature ofthe invention is the provision of an impulse member which is directlyresponsive to the tube surface temperature and is so connected to thecontrol means for the heating medium as to prevent the tubes from beingoverheated.

As is diagrammatically indicated in Fig. 4, the impulse member ispreferably a bimetallic therrnoelement '71 of any known design. thejunction of the two metals being applied to the surface of one of thetubes of the upper cage 41 adjacent the upper end thereof and theelectrical bimetals being passed outwardly through a channel 73 in thewall 7. The thermoelemcnt 71 and the leads 72 are preferably disposed onthe rear or outer side of the tube so as to be protected as much aspossible from the radiation heat. The leads 72 connect the element 71with a relay 74 which transforms, `in any conventional way, the electriccurrent produced by the heat in the bimetallic joint into mechanicalpower for regulating the lll control `means iof-theheating medium, hereillustrated by the damper 75.

.Instead-0f, .orin .addition to, the thermoclement 71, a lightsensitive-photoelectric cell 77 may be directed to ward `one of the`tubes 6 through an opening 78 in the wall 7 and the electric currentproduced thereby may be passed lthrough leads 79 to the relay 74. As thelight emission of the V.tube is `proportionate to the temperaturethereof, such a photoelectric `cell will be directly responsive tothetemperature of .the tube.

There is thus provided by the present invention an improved gas heaterofthe radiation type which is more efficient as a heat exchanger, iscapable of using higher heating gas temperatures, and is more practicaland economical to construct than similar devices previously known totheart. Although two specifically different forms of heater embodying theinvention have been described and illustrated in the accompanyingdrawing, it will be obvious -that ,the invention is not limited to theexact structures shown, but that various changes, which will now suggestthemselves to those skilled in the art, may be made in the form, detailsof construction and arrangement of the parts without departing from theinventive concept. Reference is therefore to be had to the appendedclaims for a definition of the limits of the invention.

This application is a continuation-impart of Serial No. 514,474, filedDecember lr6, 1943, now abandoned.

Alternatively or in addition to controlling the heating medium theimpulse members 71 and 77 may with advantage be connected by vmeans ofleads 81 and 82, respectively, to another relay 83 actuating a damper B4in the inletconduit for the gas to be heated.

What is claimed is:

l. A radiation gas heater comprising a wall forming a vertical shaft`and having an inner lining `of refractory material, a .head-unitcovering the upper end of said shaft and forming a heating gas passagecoaxial with and opening vertically into the central portion of saidshaft, means closing `the bottom end of said shaft and forming a secondheating gas passage coaxial with and opening vertically into saidcentral portion `of the shaft, an annular upper head box adjacent theupper end of said shaft, an annular lower head box within said shaft, aset of substantially vertical tubes arranged in a single ring about theaxis of said Shaftat a spacing between adjacent tubes of at least onehalf the tube diameter and such that radiation from the heating gas maypass between said tubes to the refractory lining of said shaft wall forreradiation thereby to the adjacent surfaces of said tubes, said tubesbeing substantially straight throughout their entire length andconnected in parallel to the bottom side of said `upper head box and tothe upper side of said lower head box and so positioned within the shaftthat they are subject to radiation from the heating gas substantiallythroughout their entire length, said head boxes and said 4tubes formingan integral `tube-cage having a central unobstructed space throughoutits entire length coaxial with said heating gas passages, gas conduitsconnected to said head boxes for supplying gas to be heated to one ofthehead boxes and discharging heated gas from the other head box, and meansassociated with said shaft wall for supporting said upper head box, saidtubes and said lower 'head ibox being suspended in the shaft from saidupper head box and freely movable in a vertical direction inbresponse tothermal expansion and contraction of said tu es.

2. A gas heater-.as defined in claim l wherein the heating gas inlet vto.the Vunobstructed space and the inlet to the tube-.cage of the gas tobe heated are located at the same Vend of the tube-cage, 4so 4that `theheating gas and the lgas to be heated flow Tin parallel through theheater, and the .length ofthe tubes is less than three times thediameter 4of the Aring'in which they are arranged.

3. A gas heater as defined Vin claim l wherein the 9 wall of said shaftis so constructed as to form a shelf over which a substantial portion ofthe bottom of the upper head box of the tube-cage projects.

4. A gas heater as defined in claim 1 wherein said supporting means forthe upper head box are so yconstructed and arranged as to form anairtight engagement between said upper head box and said shaft wall.

5. A gas heater as defined in claim 1 wherein said tube-cage is soconstructed and arranged as to be insertable in and removable from saidshaft as a unit.

6. A gas heater as defined in claim 1 wherein the diameters of said ringof tubes and said lower head box are less than the inside diameter ofthat portion of said shaft wall which surroundes said tubes and lowerhead box.

7. A gas heater as defined in claim 5 wherein the diameter of said lowerhead box is less than that of said upper head box and the tubes convergetoward their lower ends so as to form a downwardly tapering tube-cage.

8. A gas heater as defined in claim 5 wherein the means for dischargingthe heated gas comprises at least one conduit connected to thebottomof'said lower head box and passing vertically through said bottomclosure means.

9. A gas heater as defined in claim 1 wherein the means for supplyinggas to be heated comprises an inlet conduit connected to said upper headbox. y

10. A gas heater as defined in claim l including an inlet conduit forheating gas connected to the heating gas passage of said head-unit.

11. A gas heater as defined in claim 1 wherein said head-unit includes aburner for combustible fluid connected to said heating gas passage and acombustion chamber between the inlet of said burner and the mouth ofsaid heating gas passage, said burner being so constructed and arrangedas to provide substantially complete combustion within said combustionchamber.

12. A gas heater as defined in claim lincluding an inlet conduit for gasto be heated connected to said upper head box and an inlet conduit forthe heating gas connected to the gas passage of said head-unit.

13. A gas heater as delined in claim 1 wherein said head-unit isremovableas a unit from the shaft and is supported at least indirectlyon said shaft wall.

14. A gas heater as defined in claim 13 wherein said head-unit issupported directly by said upper head box.

15. A gas heater as defined in claim 14 wherein said head-unit rests bygravity on said upper head box and is in airtight engagement therewith.

16. A gas heater as defined in claim 1 wherein a substantial portion ofthe bottom of said upper head box projects over and s shielded fromradiation by a portion of said shaft wall.

17. A gas heater as defined in claim 16 wherein said upper head box isprovided with a at bottom and said tubes are connected to the innerperipheral portion of said bottom.

18. A gas heater as defined in claim l including a second tube-cagesuspended in the shaft below said first named tube-cage, said secondtube-cage comprising an upper head box, a lower head box and a set ofsubstanover the mouths of said tubes and the adjacent portions of thewalls of `said head box at a relatively small distance from said wallsto form a narrow passage between said plate and said walls leading tosaid tube mouths.

22. A gas heater as defined in claim 1 wherein at least one of said headboxes is so positioned as to expose a portion of its walls to saidheating chamber andhas a plate therein extending over the mouths of saidtubes and along said exposed portion of the walls of said box at arelatively small distance from said walls so as to form a narrow passagebetween said plate and said walls leading to said tube mouths.

References Cited in the file of this patent UNITED STATES PATENTS171,844 Payne Jan. 4, 1876 Y 613,332 Temple Nov. 1, 1898 677,798 fRiblet July 2, 1901 1,599,613, Fahrenwald Sept. 14, 1926 1,662,142 BraunMar. 13, 1928 1,671,686 Smoot May 29, 1928 1,717,334 DeFlorez June 11,1929 2,119,817 Keller June 7, 1938 2,201,616 LaMont May 21, 19402,213,324 Niemitz Sept. 3,. 1940 2,276,527 'Ihrockmorton Mar. 17, 19422,320,911 Cooper June 1, 1943 2,409,801 Ruegg Oct. 22, 1946 2,421,387Lysholm June 3, 1947 2,524,637 Ruegg Oct. 3, 1950 2,544,600 Keller Mar.6, 1951 2,555,552 Litell June 5, 1951

